HDF5 is a container format designed for big data applications. In it, vast amounts of heterogeneous data can be stored in a small number of files that are easy to manage. Detectors of the EIGER series write datasets thousands of images big to HDF5 files and record most of the metadata that are required for data processing. The metadata are saved in a master file that is separate from the data but links to it. In this talk, I will present the HDF5 format and some of the metadata as written by EIGER detectors. I will also discuss metadata that are essential for processing but unknown to the detector and highlight blank fields that the EIGER HDF5 template provides for completion by beamline routines. A related talk by Herbert J. Bernstein will explore ways of recording the geometry of the experimental setup. Software development, data processing, and effective archiving will all benefit from strict adherence to standards set by the NeXus committee.